Deflagration Flame Research Articles

Influence of Different Bifurcation Angles on the Flame Propagation of Gas Explosions in Three-Way Bifurcated Pipes

Exploring the flame propagation law in the process of gas explosion under different bifurcation angles is of great significance to the design of coal mine roadway and the prevention of gas explosion accidents. To study the variation of flame propagation law with bifurcation angle, an in-house experimental system based on a small scale three-way bifurcated pipe was developed to perform gas explosion experiments using mixtures of premixed methane-air with a methane concentration of 9.5%. Numerical simulations were conducted to study the propagation of the explosion flame. The results show that, (i) during the flame propagation process, the flame morphology evolves in the following manner: hemispherical, concave entrainment-deformation-flattening; (ii) in the case of gas explosion of three-way bifurcated pipes, there are significant differences in damage at different positions, and the damage at the pipe connection is the most serious. (iii) Although the parameters of the explosion flame in the bifurcated pipe exhibit similar trends across four different bifurcation angles, the values of the flame parameters obtained by the experiments and numerical simulations were not completely consistent. (iv) When the bifurcation angle is between the 45 and 75° bifurcation range, the area of the turbulent vortex formed by the air flow increases as the angle of the pipe widens. The research results analyze the propagation law of gas deflagration flame in the bifurcated pipeline, providing reference for the propagation mechanism of gas deflagration in underground bifurcated roadway and the formulation of prevention measures, which is conducive to preventing the propagation of gas explosion, reducing the intensity, and reducing the loss caused by gas explosion. However, large-scale tests are needed to determine the applicability of small-scale tests and calculations in this paper to full-scale mine conditions.

Study of the process of deflagration flame behavior in disturbance conditions

To evaluate the effect of the disturbance airflow on the methane deflagration flame, a deflagration experiment is conducted in a closed duct. The effects of three disturbance airflow levels and two disturbance positions on the deflagration flame dynamics, including the variation of flame propagation behavior, disturbance mechanism, and deflagration overpressure development are investigated. The experimental results show that the vertical disturbance of the airflow disrupts the original structure of the deflagration flame, increasing the inherent instability and turbulence of the flame. The increase in the initial pressure of the disturbance airflow can increase the flame turbulence intensity, propagation velocity, the maximum value of the deflagration overpressure and the rate of overpressure rise. The contribution of the disturbance airflow at the initial acceleration position of the flame to the characteristic parameters of deflagration is more prominent. The experimental results complement and explain the propagation behavior of deflagration flame in a non-static environment. It provides suggestions on the prevention and protection of combustible gas deflagration risks in complex environments.

Influence of gasoline fuel formulation on lean autoignition in a mixed-mode-combustion (deflagration/autoignition) engine

Stoichiometric spark-ignition engines suffer efficiency penalties due to throttling losses at low loads, a low specific-heat ratio of the stoichiometric working fluid, and limits on compression ratio due to end-gas autoignition leading to undesirable knocking. Mixed-Mode Combustion (MMC) mitigates these shortcomings by using a lean working fluid where a spark-initiated pilot-stabilized deflagrative flame front is followed by controlled end-gas autoignition. This MMC study investigates the effects of initial conditions (intake air temperature, intake pressure, equivalence ratio, and intake oxygen fraction) on autoignition tendency of four gasoline-range fuels with varying properties and composition. The use of fuels with varying octane sensitivity (S) allowed exploring the importance of low-temperature heat release in triggering autoignition. Fuels with high S were less reactive for conditions that promote low-temperature chemistry (operation at high intake air pressure or without N2 dilution). Conversely, an Alkylate fuel with low S showed a greater autoignition resistance at operating conditions that were unfavorable for low-temperature chemistry. Next, the effect of residual gas composition on autoignition tendency of fuels was examined with a chemical-kinetics model. Among the various molecules in the residual gas, nitric oxide (NO) enhanced the low-temperature chemistry and increased the autoignition tendency most significantly. The fuels’ autoignition response to increasing NO amount corroborates the experimental observations. Next, the sequential autoignition of the end-gas was assessed to be less impacted by thermal stratification because of lean mixtures showing relatively less low-temperature chemistry, when compared to stoichiometric mixtures. Next, the effect of changing equivalence ratio on the autoignition was found to be similar for all fuels, regardless of their S. With changing intake air temperature, the response of fuels’ autoignition tendency depended on the dilution level used. At high dilution (i.e. low intake [O2]), fuels’ reactivity increased with increasing intake air temperature. In contrast, for operation without dilution, the autoignition tendency of the low-S Alkylate fuel decreased with increasing intake air temperature, while that of high-S High Cycloalkane fuel still increased with increasing intake air temperature. In conclusion, conventional octane metrics (RON and MON) have utility in assessing the autoignition tendency under lean MMC operation. Moreover, the fuel requirements for MMC align with that of stoichiometric operation: i.e., high RON and high S fuels are desirable for stable non-knocking operation.

Deflagration evolution characteristic and chemical reaction kinetic mechanism of JP-10/DEE mixed fuel in a large-scale tube

As a high energy density hydrocarbon fuel, JP-10 is widely used in various types of engines. However, JP-10 also has problems such as not easy to ignite and incomplete combustion. DEE with good flammability and volatility can make up for the above shortcomings. In present work, a series of experiments were carried out to study the deflagration propagation characteristics of JP-10/DEE mixtures with different fuel ratios in a large-scale tube. The initial pressure wave induced by ignition and the deflagration flame front divide the flow field into an unreacted zone, a disturbance zone and a product zone. The propagation velocity of the flame front in the observation window is obtained by the Canny edge extraction procedure, and a secondary flame phenomenon is found as JP-10 slowly evaporates and ignites. With the increase of DEE concentration, the propagation speed of primary flame increases, the induction time of secondary flame is advanced. In addition, JP-10/DEE mixed fuel deflagration has a synergistic enhancement effect, and the maximum deflagration intensity is achieved when fuel ratio is 0.6. The maximum flame temperature of JP-10/DEE mixed fuel deflagration is 1660 °C, and the fastest flame propagation speed is about 97 m/s. Moreover, the adiabatic flame temperature of the JP-10/DEE gas-phase mixture decreases with the increase of DEE concentration, but the system is more easily ignited, and the emission of incomplete combustion products decreases.

Study on inhibition effect and mechanism of micron and nano Al(OH)3 powders on deflagration flame of poplar dust

In this paper, the Hartmann experimental system was used to study the effect of different proportions of micron and nano Al(OH)3 powders on the deflagration flame propagation of poplar dust. In the course of the experiment, a high-speed camera was used to record the flame shape and characteristic parameters in the process of poplar dust deflagration flame propagation. Combined with the SEM images of poplar dust explosion products under different conditions, the pyrolysis characteristic curves of poplar dust, micron and nano Al(OH)3 powders, the inhibition mechanism of micron and nano Al(OH)3 powders on poplar dust deflagration was analyzed. The experimental results show that micron and nano Al(OH)3 powders have obvious inhibition effect on poplar dust deflagration flame, and better inhibition effect can be achieved when 35% nano Al(OH)3 powder is added. And under the condition of adding the same proportion of micron and nano Al(OH)3 powders, the inhibition effect of nano Al(OH)3 powder is better than that of micron Al(OH)3 powder.

Suppression of deflagration flame propagation of methane-air in tube by argon gas and explosion-eliminating chamber

To explore the inhibitory effect of argon gas and explosion-eliminating chamber on methane-air deflagration flame propagation in the tube, based on the Φ = 120 mm, L = 5.5 m stainless steel pipeline test system to measure methane-air deflagration flame structure, flame propagation speed, and deflagration pressure. The results show that: 10–30% argon is mixed into the methane-air premixed gas with different equivalent ratios. With the increase in the mixed argon content, the tensile distortion and instability of the flame front increase, and the average value of flame propagation speed decreases by 2.52–60.0%. The first and second deflagration pressure peaks are reduced by about 13.1–62% and 17.7–86.5% respectively. The average value of the methane-air deflagration flame propagation velocity was reduced by 5.7–37.0% with the explosion-eliminating chamber laid at the nozzle. The second and third deflagration pressure peaks are reduced by about 10–30% and 50–90% respectively. The inhibitory effect of argon on the propagation of methane-air flame is considered better than the laying of the explosion-eliminating chamber under the experimental conditions.

Propagation characteristics of turbulent deflagration in horizontal tunnel with lateral liquefied petroleum gas concentration variations

The deflagration of premixed gas with a non-uniform spatial distribution is one of the critical characteristics observed in deflagration accidents in tunnels. The concentration gradient of premixed gas in the propagation direction of the deflagration flame in the tunnel is an important factor influencing the propagation of deflagration. In this study, the numerical simulation of the deflagration process in two adjacent areas with different initial concentrations of premixed liquefied petroleum gas (LPG) was implemented in a tunnel with a unidirectional opening. The dynamic coupling characteristics of the lateral concentration difference of the premixed gas in the tunnel, the deflagration propagation process, and the concentration gradient mechanism that promotes or inhibits the initial deflagration propagation were analysed. Staged violent deflagration can be induced by a sharp transition from weak initial deflagration under an appropriate concentration gradient to unforeseen severe deflagration overpressure destruction and flame propagation. In a tunnel with a large cross-sectional area, the difference in the concentration of premixed gas along the propagation path of the deflagration flame can also cause the local protrusion of this front, thereby increasing the spreading ability of deflagration flame at the local location. However, an excessively high concentration gradient may hinder the initial propagation of deflagration to a certain extent, limiting the expansion space of combustion heat and reducing the propagation speed of a flattened deflagration flame front. To characterise the propagation velocity of deflagration flame in the tunnel formed by two zones with different initial premixed LPG concentrations, a dual-zone model that combines power and exponential functions was proposed. The validity of the model was also verified.

Modeling of turbulent deflagration behaviors of premixed hydrogen-air in closed space with obstacles

The propagation of premixed hydrogen-air deflagration flames in a closed duct with different shapes of obstacles was investigated using large eddy simulation (LES). The turbulent flame wrinkling factor in the LES subgrid turbulent combustion model is dynamically modeled based on Charlette's power-law model. The LES results obtained by the dynamic flame surface density (DFSD) model can accurately match the experimental data quantitatively and qualitatively. Numerical results show that the triangular obstacle induces a higher peak overpressure, 7% and 30% higher than that in the square and circle, respectively. The formation of juxtaposed tulip flames is discovered, and the topological analysis of the velocity vector field reveals that the vortex at the tail of the obstacle is the main inducing factor for its formation. Additionally, the Karlovitz number is used to quantify the degree of turbulence-flame interaction, and the transition of deflagration flame from "wrinkled flame" to "thin reaction zone" is observed. The research helps to understand the mechanism of deflagration flame propagation induced by obstacles and provides critical information for safety planning and explosion protection.

Study on the suppression mechanism of (NH4)2CO3 and SiC for polyethylene deflagration based on flame propagation and experimental analysis

The polyethylene (PE) industry is in a stage of rapid development. However, PE dust floating in the air is frequently associated with unexpected explosions. To find a proper way of preventing and mitigating the potential of PE dust explosion, a study was carried out in which (NH4)2CO3 and SiC were used for the first time to suppress PE deflagration flames. In a Hartmann tube apparatus, PE deflagration flame propagation in the presence of (NH4)2CO3 and SiC powders as the suppressant was observed. The suppression effects of these two powders at different mass fractions were compared. The results showed that flame propagation velocity and acceleration decrease with increasing mass fraction of (NH4)2CO3 and SiC. Through XRD, FIRT, TG-DSC, GC–MS and sensitivity test, explosion products and combustion reactions were characterized and the PE explosion suppression mechanisms of (NH4)2CO3 and SiC for PE were compared and investigated. PE explosion is suppressed by SiC physically by reducing temperature and pressure of explosion combustion, while the explosion combustion of PE was suppressed by physical and chemical effect of (NH4)2CO3. As a result, (NH4)2CO3 is better able to suppress explosion flames. The study provides a theoretical and practical basis for the prevention and control of explosion fire accidents in the polyethylene industry.

Comprehensive investigations on the explosion suppression of biomass fuels: Starch as a representative

Starch is one type of biomass fuel raw material commonly used, but presents deflagration hazards when suspended in the air. Dust deflagration accidents might be effectively inhibited by creating a gas inerting atmosphere. In this work, the evolution of starch deflagration flame in the inhibitory gas atmosphere N2 or CO2 was investigated. Under the action of inerting gas, heterogeneous combustion is strengthened, the flame propagation velocity is reduced and the acceleration trend is smooth. The maximum flame temperature is decreased and fluctuation after reaching the maximum value is weakened. According to the micro characterization of deflagration residues, after injecting inhibitory gas, more amorphous carbon is protected and the carbon layer formed has a smaller crystal size, some easily oxidized functional groups in starch are maintained during the deflagration. Meanwhile, carbon content and the original chemical bond ratio in the residues are also increased. In addition, experimentally measured the maximum limit inerting ratios of CO2 and N2 are chosen as 33% and 54%, respectively. On this basis, safety application for gas inerting technology in practical processes was discussed.

Promoting of foam copper with chaotic distributed pores on the deflagration of premixed hydrogen-air flame in the tube

The deflagration flame of stoichiometric hydrogen-air mixtures is studied in this paper. Combined with the foam copper structure, the deflagration characteristic is analyzed in this paper. The experimental results show that the foam copper can improve the flow instability at the flame front and the degree of turbulence in the propagation process, the flame propagates more rapidly in turbulence state, and the number of stages of flame morphology during the propagation process in-creases. The classic ?tulip? flame can be transformed into a distorted ?tulip? flame and a fractal ?tulip? flame before it collapses. When the flame passes through the foam copper, the flame front velocity increases as the number of structural layers increases. The flame front velocity propagates at supersonic speed through the accumulation of three layers of foam copper. The instability of overpressure in the propagation process will cause oscillation. More layers of the structure, the oscillation frequency, and the amplitude of the overpressure are increased significantly. Foam copper structure has a reverse action on over-pressure. When the overpressure value is low in the early stage, the structure promotes the propagation, but the overpressure value is large in the later stage, the structure has a blocking effect.

Study on the Dynamic Process of Premixed Hydrogen-Air Deflagration Flame Propagating in a Closed Space with Obstacles

Study on the Dynamic Process of Premixed Hydrogen-Air Deflagration Flame Propagating in a Closed Space with Obstacles

Effects of typical inhibition materials on the flame propagation of pulverized coal mixture

Inert powders played an important role in the explosion inhibition of dust removal equipment and coal gasification process. In this paper, SiO2, dry water, carrageenan dry water, and NaHCO3 were adopted to explore the influences of inert powders on the deflagration flame propagation characteristics of mixed pulverized coal by a vertical pipeline. The results showed that the flame brightness of the mixed pulverized coal decreased with the irregularly discrete of flame front after the addition of inert powders. As the increasing of pulverized coal mesh number, the peak flame temperature first increased and then decreased, while the drop value of peak flame velocity and temperature increased with the increasing of mass concentration. In addition, the inerting rate affected the propagation of pulverized coal composite flame. When the inerting rate exceeded 1.0, the peak temperature drop increased rapidly and dry water exhibited the best inhibition performance.

Large Eddy Simulation of Premixed CH4/Air Deflagration in a Duct with Obstacles at Different Heights.

In this work, the propagation of CH4/air deflagration flames in three semi-confined ducts with different obstacles was numerically investigated using large eddy simulation (LES). The shape of the premixed flame, flow field structure, and overpressure characteristics of the interaction between the flame and the obstacle are simulated accurately in three ducts with obstacles of different heights. The results show that the structure of the flame is changed by the presence of obstacles, and a change in the shape of the hemispherical conical brush appears, and a flame vortex is generated by the entrapment of unburned premixed gas on the left side of the obstacles. In the process of CH4/air deflagration, the existence of obstacles would lead to the change in combustion velocity and overpressure relief velocity and then have a certain influence on the peak of overpressure and the shape of the premixed flame.

Experiment on influence of inert powder on deflagration of oil shale dust research

AbstractTo study the effect of inert powder on the deflagration of oil shale dust, the Hartmann tube experiment device and 20 L spherical explosion experiment device were used. The effects of three kinds of inert powder (KH2PO4, NaHCO3, Al(OH)3) and widely used rock powder (CaCO3) on the deflagration of oil shale dust in different proportions were studied. The results show that the four inert powders can effectively dilute the concentration of oil shale dust cloud, slow down the reaction rate, delay the flame propagation, and reduce the maximum flame propagation speed. When 50 wt% KH2PO4, NaHCO3, CaCO3, Al(OH)3 inert powder is added, the maximum flame propagation speed is 44.14%, 41.11%, 31.79%, 58.96%, and the maximum explosion pressure drop is 70.0%, 66.0%, 59.5%, 77.0%. When Al(OH)3 was added at 10 wt%, 30 wt%, and 50 wt%, the combustion time of oil shale was extended to 162.7, 223.4, and 321.5 ms, respectively. With the addition of Al(OH)3, the time from ignition time to maximum explosion pressure of oil shale dust increases gradually, the maximum flame propagation distance is shortened obviously, the maximum flame propagation speed and maximum explosion pressure drop the most, and the effect of inhibiting oil shale dust deflagration flame is the best.

Hindrance and suppression characteristics of local whole-inerting zone on flame propagation of methane/coal dust deflagration

To investigate the suppression of local whole-inerting on the flame propagation of methane/coal dust deflagration. N2 was used as the loading gas to dispersed the fly ash into a fixed zone of the pipe and thus form local whole-inerting zone (contained N2-Fly ash), and the widths of inerting zone were determined by theoretical calculation. The deflagration experiments under different concentrations of fly ash in inerting zone and different widths of inerting zone were conducted by using vertical combustion pipe. Results indicate that the flame maximum height, bottom velocity and temperature of methane/coal dust deflagration were decreased obviously with the fly ash concentration and inerting zone width increased. The inerting zone aggravated the attenuation amplitude of flame velocity at deceleration period, and the velocity difference (Δv) between the maximum velocity and the bottom velocity was increased obviously. These combinations (concentration, width) can suppress significantly the deflagration flame propagation: (100 g/m3, 30 cm), (200 g/m3, 25 cm), (300 g/m3, 25 cm), (400 g/m3, 25 cm), (500 g/m3, 20 cm) and (600 g/m3, 15 cm). The inerting zone can suffocate the deflagration flame, and contained N2 and fly ash which can hinder the coal decomposition and the combustion reaction through dilution effect, heat absorption, heat insulation and free radicals consumption, so that deflagration reaction and flame propagation were weakened.

Influence of water storage on deflagration characteristics of methane in confined space

Comparative experiments were conducted under different water level heights and methane concentration conditions using a self-designed explosion experiment pipeline. The results showed that, in comparison with the scenario without water storage, when the water level was 2 cm and methane concentration was 6.5–12.5%, there was a dual effect including a pressure decrease caused by the endothermic cooling of liquid water and a pressure increase caused by the expansion of water vapour. These effects caused the pressure time history curve to exhibit a double-peak or multi-peak structure, and the average decrease in the peak deflagration pressure was 23.76%. The heat of vaporisation absorbed by the stored water and barrier effect of water vapour on the transfer of the heat slowed down the increase in the deflagration temperature. The average decrease in the peak deflagration temperature of methane was 13.82%, and the time to reach the peak deflagration temperature was extended as a whole, with an average delay of 0.22 s. Water storage also changed the shape of the deflagration flame front, which exhibited ‘knife’, ‘V’, and ‘crescent’ structures. Moreover, the flame propagation speed was significantly reduced, with the peak and average flame propagation speeds decreased by 83.3% and 83.6%, respectively. The research results can provide a certain reference for preventing gas explosions in typical confined spaces, and also help to explore new anti-explosion methods, which can be applied to marine equipment such as ships.

Suppression of methane/coal dust deflagration flame propagation by CO2/fly ash as a flue gas layer

To reveal the suppression characteristics of methane/coal dust deflagration flame propagation by the flue gas loading in local zones. The suppression experiments of flue gas layer (CO2 and fly ash) with different thicknesses and fly ash concentrations were conducted in semi-open vertical combustion pipe. The flue gas layer was produced by self-designed flue gas layer generator. Flame propagation characteristics which including the flame image, velocity, the ion current and temperature were recorded by the high-speed photography, the ion current probe and the thermocouple. The residues after coal dust deflagration were characterized by FTIR (Fourier transform infrared spectroscopy). The results show that the flue gas layer has a significant suppression on deflagration flame propagation. With the increasing of flue gas layer thickness and fly ash concentration, the flame velocity, height, temperature and the ion current gradually decreases, and the suppression effect was enhanced. The asphyxiation of CO2, heat absorption and insulation of fly ash were mainly methods for the suppression of coal dust deflagration flame.

The control mechanisms of lysine sulfate dust deflagration flame propagation based on flame microstructures and time scale analysis

A series of experiments of the deflagration flame propagation of Lysine sulfate (LS) powder were conducted in a vertical tube with different moisture contents, particle sizes and mass concentrations. The thermal flame model was applied to investigate the dust flame micro-structures combined with the ion probes and the fine thermocouples. The time scales of the dust combustion sub-processes were calculated to reveal the control mechanisms during flame propagation. Results showed that a small amount of water significantly reduced the flame thickness and the heat transfer time, like the SEEP effects (Suppressant Enhanced Explosion Parameter). The practicability of applying the thermal flame model for dust flame was proved. As the mass concentration increased, the effect of the pyrolysis enhanced initially and then decreased during the gas phase combustion process, and the effect of heat transfer weakened during the pyrolysis process.

Laboratory investigation of powder deflagration flame propagation and quenching behaviors suppressed by ZrO2 ceramic foam

Ceramic foam, with its complex three-dimensional open porous structure, is a promising material for explosion suppression. To assess the suppression effect of ceramic foam on powder deflagration flames, tests were conducted in a vertical quartz tube. Results show that the position of ZrO2 ceramic foam affects coal powder flame propagation behaviors. Compared with conditions in an empty tube, ceramic foam can reduce the pulsation amplitude of flame propagation velocity and acceleration, but ceramic foam which is very close to the ignition source will increase the pulsation amplitude in the initial stage of deflagration to a certain extent. The ceramic foam has a good quenching effect on powder deflagration flames. Ceramic foam can dissipate deflagration energy and block burning and unburned powder, so that the flame will be interrupted due to the loss of fuel supply. The results are of significance to the prevention and control of powder deflagration hazards.